In radial kerototomy surgery it is extremely important the percise depth of the incision be predeterminable. Specifically, a cut to depths deeper than intended may cause loss of eye fluid and permanent loss of sight. It is also desirable that different cuts be made to different depths, which depths must be accurately controlled to prevent the aforementioned undesirable effects of too deep an incision. Therefore, for radial kerototomy procedures, it is important to provide a surgical knife with a precisely controlled mechanism for exposing a predetermined blade depth. It is also important to assure that no more of the blade than the exposed predetermined depth be permitted to penetrate the eye. It is likewise important that such a knife arrangement be configured so that the incision, or a portion thereof, is not obscured by the knife body.
The prior art includes U.S. Pat. No. 3,967,377 to Wells, wherein a spindle is threadedly received in the knife body and, in turn, threadedly receives, in a common direction, a blade supporting chuck which is restrained from rotating relative to the body. The body thread pitch is greater than the chuck thread pitch so that, as the spindle is rotated into the body, the chuck retracts into the spindle by a lesser distance. The overall effect is a translation of the chuck by an amount proportional to the difference between the thread pitches. The arrangement provides precision control over blade movement; however, no adequate means is provided to preclude inadvertent insertion of the blade to a depth beyond the intended incision depth. Moreover, in the Wells device, movement of the blade back and forth relative to the body renders the blade subject to misalignment relative to the body member.
On the other hand, U.S. Pat. Nos. 4,026,295 to Lieberman and 3,945,117 to Beaver, disclose guards or ski members through which the blade projects and which rests against the eye surface to prevent inadvertent insertion of the blade to a depth greater than the exposed length of blade. However, the adjustment for blade length exposure in these devices is not very precise and the nature of the guard or ski structure precludes substantial improvement in this regard. Moreover, the bulky knife body requirements of these structures, and the bulky guard and ski structures, limit the surgeon's visibility of the incision without awkward positioning of the knife.